Jet mills are known for performing micronisation of powder or particle material. Different kind of jet mills are available such as for example fluidized bed jet mill or spiral jet mill.
FIGS. 1A and 1B relate to conventional air spiral jet mills. They consist of a flat, cylindrical grinding chamber (3), into which the grinding gas flows tangentially by way of multiple grinding nozzles (2). The product to be ground is fed in by way of the opening (4) and the grinding gas is fed in under pressure into the collecting opening (1). The grinding gas is distributed among the individual nozzles (2), which are distributed over the circumference of the mill, and blown into the grinding chamber (3). The nozzles (2) are pre-set at a suitable angle which creates a spiral flow in the grinding chamber (3). The material to be ground is transported for example by means of a feed screw from a hopper into the grinding chamber and mixed there with the grinding gas to form a stream of gas and solid matter and is accelerated in the grinding chamber. The ground particles are discharged by way of classifier (5).
All known processes for micronisation have the objective of achieving the greatest possible throughput with the smallest possible fineness particle. The processes that are designed purely for cost-effectiveness largely disregard the morphological properties of the particles, such as the particle form and the specific surface area. These properties are of importance as they largely influence the later rheological and physical characteristics of the particle. Thus, in the case of graphite for example, the electrical and the thermal conductivity can be influenced. By producing carbons in flake form with a smooth surface, it could be possible for example to improve the reflectivity, and to use the flakes as a pigment or as an athermanous material in heat insulating systems.
The nature of the collision between particles, the acceleration energy and the chemical-atmospheric conditions in the grinding chamber are of importance. Therefore, numerous methods have been proposed for changing the grinding conditions in jet mills.
It is known from DE 7617063 that the angles and the arrangements of the nozzles are of importance and a nozzle ring has been proposed for easier exchange, better cleaning and noise damping. However, the particle morphology cannot be influenced in this way.
It is proposed in WO 2008046403 to use steam, hydrogen gas or helium gas as grinding gases as they can reach a higher sonic velocity than air (343 m/s) as the fluid. A velocity of at least 450 m/s is disclosed. On account of the different densities and viscosities of the gases, the gas velocity can be significantly increased, but cost-effective grinding operation is no longer possible in terms of safety and costs, at least in the case of hydrogen gas and helium gas. No teaching is given regarding a possible influence on the particle morphology by the cited grinding gases.
To increase the throughput capacity, grinding aids have also been proposed. The use of grinding aids is sufficiently well known. According to the present state of the art, they are used primarily for increasing the throughput in ball mills, as is the case for example when grinding clinker/cement. Most of the known grinding aids concern wet grinding in ball mills or in agitator ball mills. The grinding of graphite in a liquid is described for example in U.S. Pat. No. 4,533,086.
Glycols, glycol derivatives, carboxylic acids and wetting agents are described for example in US 2012304892 and in WO 2011161447. However, these additives concern the grinding of clinker or cement and are not used in jet mills.
EP 1015117 describes the use of different grinding gases, but merely involves drying a slurry by means of a vortex mill.
Therefore, there is still a need for a cost-effective process for grinding particles in dry grinding processes in which fine and morphologically optimized particles are produced.
It is an object of the present invention to provide a process and an apparatus for grinding particles in a cost effective way in which fine and morphologically optimized particles with a reduced specific surface area and/or with high aspect ratio are produced.
It is an object of the present invention to provide a process and an apparatus to improve the particle morphology in a cost-effective way.
It is also an object of the invention to provide fine and morphologically optimized particles, i.e. micronized particles with a reduced specific surface area and/or high aspect ratio.